With reference to FIGS. 8 and 9, a description is given of operations performed when a terminating-call signal for telephony service is received in a mobile communication system where a 2G/3G scheme and an LTE (Long Term Evolution) scheme coexist.
As shown in FIG. 8, in Step S2001, a server S-CSCF (Serving Call Session Control Function) receives “INVITE” which is a SIP (Session Initiation Protocol) signal as a terminating-call signal for telephony service. The server S-CSCF then sends an application server AS this “INVITE” in Step S2002.
In Step S2003, the application server AS determines the terminating domain of the “INVITE”. Operations of this determination are described below referring to FIG. 9.
Specifically, as shown in FIG. 9, upon receipt of the “INVITE” in Step S2002, the application server AS sends a subscriber information server HSS (Home Subscriber Server) a status inquiry request in Step S2003a. 
The subscriber information server HSS sends a switch SGSN (Serving GPRS Support Node) a status inquiry request in Step S2003b, and receives a status inquiry response from the switch SGSN in Step S2003c, the response including “IMS voice over PS session supported indication” and “Time of last contact”.
Alternatively, the subscriber information server HSS sends a switch MME (Mobility Management Entity) a status inquiry request in Step S2003d, and receives a status inquiry response from the switch MME in Step S2003e, the response including “IMS voice over PS session supported indication” and “Time of last contact.”
Here, “IMS voice over PS session supported indication” is information indicating whether or not a telephony service can be provided via IMS (IP Multimedia Subsystem) in a location registration area (RA: Routing Area or TA: Tracking Area) in which a mobile station UE is currently registered. “IMS voice over PS session supported indication” is information held by the switches SGSN/MME.
In addition, “Time of last contact” is information indicating a time of last communications between the mobile station UE and the switch SGSN/MME.
In Step 2003f, the subscriber information server HSS sends the application server AS a status inquiry response including “IMS voice over PS session supported indication” and “Time of Last Contact”
In Step S2003g, the application server AS determines the terminating domain of the “INVITE” based on “IMS voice over PS session supported indication,” “Time of last contact,” and the like.
When the determined terminating domain, of the “INVITE” is a CS (Circuit Switch) domain, the application server AS sends the server S-CSCF “INVITE” in Step S200424. Then, in Step S2005A, the server S-CSCF sends a server BGCF (Breakout Gateway Control Function) “INVITE.” In Step S2006A, the server BGCF sends a server MGCF (Media Gateway Control Function) “INVITE.” In Step S2007A, the server MGCF sends an switch MSC (Mobile-service Switching Center) an ISUP (ISDN User Part) signal as a terminating-call signal for telephony service.
When the determined terminating domain of the “INVITE” is IMS, the application server AS sends the server S-CSCF “INVITE” in Step S2004B. Then, in Step S2005B, the server S-CSCF sends a server P-CSCF (Proxy Call. Session Control Function) “INVITE.” In Step 2006B, the server P-CSCF sends the switch P-GW or a switch GGSN (Gateway GPRS Support Node) a packet signal as a terminating-call signal for telephony service.
An existing mobile communication system is configured such that, when a mobile station UE moves across a border between an area covered by an switch MME #1 (i.e., a TA in a TA list #2) and an area covered by an switch MME #2 (i.e., a TA in a TA list #3) as shown in FIG. 10(a), the switch MME #2 performs location registration of the mobile station UE at the subscriber information server HSS.
Similarly, the existing mobile communication system is configured such that, when a mobile station UE moves across a border between an area covered by an switch SGSN #1 (i.e., an RA #2) and an area covered by an switch SGSN #2, an RA #3), the switch SGSN #2 performs location registration of the mobile station UE at the subscriber information server HSS.
In other words, the existing mobile communication system is configured such that the mobile station UE does not perform location registration of the mobile station UE to the same switch SGSN/MME. The switch SGSN/MME is configured to perform location registration of a mobile station at the subscriber information server when the mobile station UE moves across a border between the ME-coverage areas.
Alternatively, the existing mobile communication system is configured such that, when a mobile station UE moves across a border between an area (RA) covered by the switch SGSN and an area (TA) covered by the switch MME, the switch SGSN/MME performs location registration of the mobile station UE at the subscriber information server HSS if ISR (Idle mode Signalling Reduction) is not provided.
Meanwhile, the existing mobile communication system is configured such that, even when a mobile station UE moves across a border between an area (RA) covered by the switch SGSN and an area (TA) covered by the switch MME, the switch SGSN/MME does not perform location registration of the mobile station UE at the subscriber information server HSS if ISR is provided.
Accordingly, as described referring to FIGS. 8 and 9, the subscriber information server HSS is configured to send the switch SGSN/MME a status inquiry request every time “INVITE” is received.